1. Field of the Invention
The present invention relates to a nitride semiconductor device.
2. Related Art
In a power semiconductor device such as a switching element or a high frequency power semiconductor device, it is effective to use a material having a high critical electric field and consequently a nitride semiconductor material having high critical electric field strength is used.
As for a conventional nitride semiconductor device using a nitride semiconductor material, a first reference art is known. According to the first reference art, a carrier transit layer formed of an AlXGa1-XN (0≦X<1) film and a barrier layer formed of an AlYGa1-YN (0<Y≦1, X<Y) film are stacked in order, a gate electrode is formed near a central part on the surface of the barrier layer having a uniform thickness, and a source electrode and a drain electrode are formed in substantially symmetric positions with the gate electrode between.
The AlN film is smaller in lattice constant than the GaN film. When the Al composition ratio of the barrier layer is greater than the Al composition ratio of the carrier transit layer, therefore, the lattice constant of the barrier layer becomes smaller than the lattice constant of the carrier transit layer and a strain is caused in the barrier layer. In the nitride semiconductor, polarization charges are generated in the barrier layer by piezo polarization and spontaneous polarization resulting from the strain in the barrier layer. And two-dimensional electron gas is formed at an interface between the carrier transit layer and the barrier layer by the polarization charges generated at this time.
For example, when a GaN film having Al composition ratio X=0 is used for the carrier transit layer and an AlYGa1-YN film is used for the barrier layer, a carrier density nS of the two-dimensional electron system is given by the following equation (1) with respect to a film thickness d1 [Å] (see, for example, J. R Ibbetson et al., “Polarization effects, surface states, and the source of electrons in AlGaN/GaN heterostructure field effect transistors,” Applied Physics Letters, 10 Jul. 2000, Vol. 77, No. 2, PP. 250-252).nS=σPZ×(1−TC/d1)[cm−2]  (1)
Here, σPZ is a charge density of polarization charges generated in the barrier layer, and d1 is a film thickness of the barrier layer under the gate electrode. TC is a critical film thickness of the barrier layer at which carriers are generated. This critical film thickness TC is given by the following equation (2), and has dependence upon Al composition ratio.TC=16.4×(1−1.27×Y)/Y[Å]  (2)
Furthermore, a second reference art is known. According to the second reference art, a recess structure is formed by removing a part of the barrier layer in order to reduce the contact resistance in the source electrode/drain electrode in the nitride semiconductor device or the gallium arsenide semiconductor device (see, for example, JP-A 2001-274375 (KOKAI), and JP-A 2004-22774 (KOKAI)). A hetero junction field effect transistor (hereafter referred to as HJFET) described in JP-A No. 2001-274375 has a structure obtained by stacking an undoped nitride aluminum (AlN) buffer layer, an undoped GaN channel layer, an n-type AlGaN electron supply layer, a Si single atom layer and an n-type GaN cap layer are stacked on a sapphire substrate in order, removing the whole of the n-type GaN cap layer and the Si single atom layer and a part of the n-type AlGaN electron supply layer in a position where the gate electrode is formed, thereby forming a recess structure, forming a gate electrode in this recess structure, and forming a source electrode/drain electrode on the n-type GaN cap layer having the gate electrode between. In this nitride semiconductor device, the contact resistance of the source electrode/drain electrode is lowered by providing the GaN cap layer between the source electrode/drain electrode and the n-type barrier layer.
A HJFET described in JP-A 2004-22774 (KOKAI) has a structure obtained by stacking a buffer layer formed of a semiconductor layer, a GaN channel layer, an AlGaN electron supply layer, an n-type GaN layer and an AlGaN layer in order on a substrate of sapphire or the like, removing the whole of the AlGaN layer and the n-type GaN layer and a part of the AlGaN electron supply layer in a position where the gate electrode is formed, thereby forming a recess structure, forming a gate electrode on the AlGaN electron supply layer in this recess structure, and forming a source electrode/drain electrode on the topmost AlGaN layer having the gate electrode between. In this nitride semiconductor device, the contact resistance of the source electrode/drain electrode is lowered by providing the AlGaN layer and the n-type GaN layer between the source electrode/drain electrode and the barrier layer.
In the nitride semiconductor devices described in JP-A 2001-274375 (KOKAI), and JP-A 2004-22774 (KOKAI), the AlGaN electron supply layer corresponds to the barrier layer and the GaN channel layer underlying the AlGaN electron supply layer corresponds to the carrier transit layer. As described with reference to the first reference art, therefore, polarization charges are generated in the barrier layer and two-dimensional electron gas is formed at an interface between the carrier transit layer and the barrier layer. However, the carrier density of the two-dimensional electron system under the gate electrode in the nitride semiconductor device having a recess structure depends upon the Al composition ratio Y of the barrier layer and the thickness of the barrier layer under the gate electrode.
A third reference art using the InYAlZGa1-Y-ZN film as the contact layer in the nitride semiconductor device is known (see, for example, “IEEE TRANSACTIONS ON ELECTRON DEVICE,” Vol. 52, No. 10, October, 2005, p. 2124). According to “IEEE TRANSACTIONS ON ELECTRON DEVICE,” Vol. 52, No. 10, October, 2005, p. 2124, a contact layer having a thick film thickness can be formed because the InYAlZGa1-Y-ZN film makes lattice matching to a GaN film when the relation Z=4.66×Y is satisfied. According to “IEEE TRANSACTIONS ON ELECTRON DEVICE,” Vol. 52, No. 10, October, 2005, p. 2124, favorable ohmic contact can be formed because polarization possessed by the contact layer and polarization possessed by the barrier layer cancel each other at an interface between the contact layer and the barrier layer and consequently depletion at the interface between the contact layer and the barrier layer can be prevented, by setting Y so as to generate polarization greater than the AlGaN layer when the relation Z=4.66×Y is satisfied.
A nitride semiconductor device having a configuration obtained by stacking a buffer layer formed of a GaN film and a barrier layer formed of an AlGaN film is known as a fourth reference art (see “IEEE TRANSACTIONS ON ELECTRONICS,” Vol. E82-C, No. 11, November, 1999, p. 1895). In the fourth reference art, the barrier layer is smaller in lattice constant than the buffer layer, and consequently strain is caused in the barrier layer. In the nitride semiconductor, two-dimensional electron gas is formed at the interface between the buffer layer and the barrier layer by a piezo effect caused by the strain in the barrier layer. Therefore, the nitride semiconductor device according to the fourth reference art can be made to operate as a field effect transistor by forming a source electrode, a drain electrode and a gate electrode on the barrier layer.
A nitride semiconductor device having a configuration obtained by stacking a buffer layer formed of a GaN film, a first barrier layer formed of an AlGaN film, a channel layer formed of a GaN film and a second barrier layer formed of an AlGaN film is known as a fifth reference art (see JP-A 2001-196575 (KOKAI)). In the fifth reference art, residual carriers in the buffer layer are prevented from influencing the channel layer by the first barrier layer. By forming a source electrode, a drain electrode and a gate electrode on the second barrier layer, therefore, the nitride semiconductor device according to the fifth reference art can be made to operate as a field effect transistor with the influence of the residual carriers in the buffer layer excluded as compared with the nitride semiconductor device according to the fourth reference art.
If in the nitride semiconductor device according to the fifth reference art a buffer layer formed of a GaN film, a first barrier layer formed of an InAlGaN film, a channel layer formed of a GaN film and a second barrier layer formed of an AlGaN film are stacked and the In composition ratio in the first barrier layer is in the range of 0.3 to 0.7, then the density of electrons stored in the channel layer can be increased by the spontaneous polarization and piezo polarization caused in the first barrier layer.
In the case of the nitride semiconductor device in which the gate electrode and the source electrode/drain electrode are formed on barrier layers having the same thickness as in the first reference art, a two-dimensional electron system having uniform carrier densities is formed at the interface between the carrier transit layer and the barrier layer when the film thickness of the barrier layer is equal to or greater than the critical film thickness TC indicated by the equation (2). Therefore, a two-dimensional electron system is formed at the interface between the carrier transit layer and the barrier layer, located between the source electrode and the gate electrode and between the drain electrode and the gate electrode as well. As a result, the on-resistance becomes low. Since a two-dimensional electron system having a finite carrier density exists under the gate electrode as well, the nitride semiconductor device becomes normally-on type.
On the other hand, when the film thickness of the barrier layer is equal to or less than the critical film thickness TC indicated by the equation (2), the carrier density of the two-dimensional electron system under the gate electrode becomes zero, resulting in a normally-off type nitride semiconductor device. However, the carrier density of the two-dimensional electron gas becomes zero at the interface between the carrier transit layer and the barrier layer, located between the gate electrode and the drain electrode and between the gate electrode and the source electrode as well besides under the gate electrode. As a result, the resistance between the drain electrode and the source electrode becomes large and the on-resistance also becomes high. In other words, as for the nitride semiconductor device according to the first reference art, it is difficult to fabricate a normally-off type nitride semiconductor device having low on-resistance with a high yield.
On the other hand, in the case of the nitride semiconductor device in which the recess structure is formed by removing a part of the barrier layer and the film thickness of the barrier layer under the gate electrode is decreased as in the second reference art, the two-dimensional electron system is formed at the interface between the carrier transit layer and the barrier layer, located between the source electrode and the gate electrode and between the drain electrode and the gate electrode and consequently the on-resistance becomes low, when the film thickness of the barrier layer between the source electrode and the gate electrode and between the drain electrode and the gate electrode is equal to or greater than the critical film thickness TC. If the film thickness of the barrier layer under the gate electrode is equal to or less than the critical film thickness TC, the carrier density of the two-dimensional electron system under the gate electrode becomes zero. As a result, the nitride semiconductor device according to the second reference art can be implemented as a normally-off type nitride semiconductor device.
Considering the energy difference in the conduction band between the carrier transit layer and the barrier layer required to implement the two-dimensional electron system, it is desirable that the Al composition ratio Y of the barrier layer is at least 0.2. At this time, it is necessary that the film thickness of the barrier layer is equal to or less than approximately 60 Å according to the equation (2) in order to make the carrier density under the gate electrode equal to zero. For implementing the normally-off type semiconductor device by using the recess structure, processing of forming the carrier transit layer, the barrier layer and the contact layer one after another by using an epitaxial crystal growth equipment and then removing a part of the barrier layer while exercising control so as to have a film thickness of 60 Å or less with high precision. Because of the problem of the processing precision, however, there is a problem that it is difficult to fabricate the normally-off type semiconductor device with a high yield.
It is known that use of dry etching such as the RIE method on the nitride causes introduction of etching damages due to the nitrogen vacancy into the semiconductor device. When processing the above-described recess structure, therefore, etching damage is introduced into a gate recess region and the channel mobility of the two-dimensional electron system under the gate recess is reduced, and it is difficult to fabricate a semiconductor device having low on-resistance. Especially in the case of the normally-off type, the film thickness of the barrier layer becomes as small as approximately several tens Å, and consequently the etched surface gets near carriers that travel through the interface between the carrier transit layer and the barrier layer and lowering in channel mobility becomes large.
The threshold voltage in the nitride semiconductor device according to the second reference art becomes (carrier density of the two-dimensional electron system under the gate electrode)/(gate capacitance per unit area). Denoting the dielectric constant of the barrier layer by ∈, therefore, the threshold voltage Vth is given by the following equation (3).Vth=σPZ/∈×(dI−TC)  (3)
In other words, the threshold voltage Vth depends upon the Al composition ratio and the film thickness of the barrier layer as indicated by the equations (3) and (2). For example, even if processing is conducted with a comparatively favorable precision so as to make the variation of the film thickness of the barrier layer under the gate equal to 10 Å in etching to form the recess structure, the variation of the threshold voltage becomes as large as 0.3 V when the Al composition ratio Y of the barrier layer is 0.3. Therefore, there is also a problem that it is difficult to fabricate a nitride semiconductor device by controlling the threshold voltage with a high yield.
In the third reference art, favorable ohmic contact can be formed by using the InYAlZGa1-Y-ZN film as the contact layer as described above. However, the third reference art has the same problem as the second reference art does. Specifically, since the structure under the gate does not change from that in the second reference art, processing of removing a part of the barrier layer 2 under the gate electrode while exercising control so as to make the film thickness of the barrier layer 2 equal to or less than the critical film thickness with high precision becomes necessary. Because of the problem of the processing precision, therefore, it is difficult to fabricate a normally-off type semiconductor device with a high yield. In the case of the normally-off type, the film thickness of the barrier layer is small in the same way as the second reference art. Since consequently the etched surface is close to carriers traveling through the interface between the carrier transit layer and the barrier layer, lowering of the channel mobility is made large by etching damage.
In the fourth and fifth reference arts, it is difficult to obtain favorable pinch-off characteristics because of causes described hereafter.
In the nitride semiconductor device according to the fourth reference art, carriers remaining in the buffer layer move to the interface between the buffer layer and the barrier layer. Even if the nitride semiconductor device is in the off-state, therefore, the leak current between the source and the drain cannot be suppressed. Therefore, it is difficult to improve the pinch-off characteristics of the nitride semiconductor device.
In the nitride semiconductor device according to the fifth reference art, polarization charges are generated in the first barrier layer as well. Therefore, two-dimensional electron gas is formed not only at the interface between the channel layer and the second barrier layer but also at the interface between the buffer layer and the first barrier layer. As a result, the pinch-off characteristics are degraded. For preventing the two-dimensional electron gas from being formed at the interface between the buffer layer and the first barrier layer, the Al composition ratio of the first barrier layer must be made small. For forming a barrier having a sufficient height, for example, for forming a barrier of 1 eV, the thickness of the first barrier layer needs to be approximately 500 nm. In general, it is difficult to stack the thick AlGaN film with a high quality because of lattice mismatching between the GaN film and the AlGaN film. Furthermore, since the Al composition ratio is low, piezo polarization generated in the first barrier layer is small and the potential in the channel layer near the second barrier layer does not change so largely. Therefore, the leak current between the source and the drain caused by residual carriers in the channel layer cannot be prevented, and it is difficult to improve the pinch-off characteristics remarkably.
In the case where an InAlGaN film having an In composition ratio in the range of 0.3 to 0.7 is used for the first barrier layer in the nitride semiconductor device according to the fifth reference art, the energy of the conduction band on the channel layer side is lowered by the polarization in the first barrier layer. Accordingly, carriers remaining in the channel layer become active. As a result, the leak between the source and the drain is caused, and consequently remarkable improvement of the pinch-off characteristics is difficult.